Silver has long been valued as a precious metal, and it is used to make ornaments, jewelry, high-value tableware, utensils (hence the term silverware), and currency coins. Today, silver metal is also used in electrical contacts and conductors, in mirrors and in catalysis of chemical reactions. Its compounds are used in photographic film, and dilute silver nitrate solutions and other silver compounds are used as disinfectants and microbiocides. While many medical antimicrobial uses of silver have been supplanted by antibiotics, further research into clinical potential continues.

Characteristics

Silver is a very ductile, malleable (slightly harder than gold), monovalent coinage metal, with a brilliant white metallic luster that can take a high degree of polish. It has the highest electrical conductivity of all metals, even higher than copper, but its greater cost has prevented it from being widely used in place of copper for electrical purposes. Despite this, 13,540 tons were used in the electromagnets used for enriching uranium during World War II (mainly because of the wartime shortage of copper).[1][2][3] An exception to this is in radio-frequency engineering, particularly at VHF and higher frequencies, where silver plating to improve electrical conductivity of parts, including wires, is widely employed. Another notable exception is in high-end audio cables, where manufacturers claim that scaling copper conductors by 6% achieves slightly better results.

Isotopes

See main article: Isotopes of silver. Naturally occurring silver is composed of two stable isotopes, 107Ag and 109Ag, with 107Ag being the most abundant (51.839% natural abundance). Silver's isotopes are almost equal in abundance, something which is rare in the periodic table. Silver's atomic weight is 107.8682(2) g/mol.[5][6] Twenty-eight radioisotopes have been characterized, the most stable being 105Ag with a half-life of 41.29 days, 111Ag with a half-life of 7.45 days, and 112Ag with a half-life of 3.13 hours. This element has numerous meta states, the most stable being 108mAg (t1/2 = 418 years), 110mAg (t1/2 = 249.79 days) and 106mAg (t1/2 = 8.28 days). All of the remaining radioactive isotopes have half-lives that are less than an hour, and the majority of these have half-lives that are less than three minutes.

The palladium isotope107Pd decays by beta emission to 107Ag with a half-life of 6.5 million years. Ironmeteorites are the only objects with a high-enough palladium-to-silver ratio to yield measurable variations in 107Ag abundance. Radiogenic107Ag was first discovered in the Santa Clara meteorite in 1978.[8] The discoverers suggest the coalescence and differentiation of iron-cored small planets may have occurred 10 million years after a nucleosynthetic event. 107Pd–107Ag correlations observed in bodies that have clearly been melted since the accretion of the solar system must reflect the presence of unstable nuclides in the early solar system.[9]

Silver oxide, produced when silver nitrate solutions are treated with a base, is used as a positive electrode (anode) in watch batteries. Silver carbonate is precipitated when silver nitrate is treated with sodium carbonate .

2 AgNO3 + 2 OH- → Ag2O + H2O + 2 NO3-

2 AgNO3 + Na2CO3 → Ag2CO3 + 2 NaNO3Silver fulminate, a powerful, touch-sensitive explosive used in percussion caps, is made by reaction of silver metal with nitric acid in the presence of ethanol . Another dangerously explosive silver compound is silver azide, formed by reaction of silver nitrate with sodium azide .[10]

Applications

Many well known uses of silver involve its precious metal properties, including currency, decorative items and mirrors. The contrast between the appearance of its bright white color to other media makes it very useful to the visual arts. It has also long been used to confer high monetary value as objects (such as silver coins and investment bars) or make objects symbolic of high social or political rank.

Currency

See main article: Silver coin and Silver standard. Silver, in the form of electrum (a gold-silver alloy), was coined to produce money around 700 BC by the Lydians. Later, silver was refined and coined in its pure form. Many nations used silver as the basic unit of monetary value. In the modern world, silver bullion has the ISO currency code XAG. The name of the pound sterling (£) reflects the fact it originally represented the value of one troy pound of sterling silver; other historical currencies, such as the French livre, have similar etymologies. During the 19th century, the bimetallism that prevailed in most countries was undermined by the discovery of large deposits of silver in the Americas; fearing a sharp decrease in the value of silver and thus the currency, most states switched to a gold standard by 1900. In the Spanish language, Plata means both silver and money.

The 20th century saw a gradual movement to fiat currency, with most of the world monetary system losing its link to precious metals after Richard Nixon took the United States dollar off the gold standard in 1971; the last currency backed by gold was the Swiss franc, which became a pure fiat currency on 1 May 2000. During this same period, silver gradually ceased to be used in circulating coins; the United States minted its last circulating silver coin in 1969.

Jewelry and silverware

See main article: jewelry and silversmith. Jewelry and silverware are traditionally made from sterling silver (standard silver), an alloy of 92.5% silver with 7.5% copper. In the US, only an alloy consisting of at least 90.0% fine silver can be marketed as "silver" (thus frequently stamped 900). Sterling silver (stamped 925) is harder than pure silver, and has a lower melting point (893 °C) than either pure silver or pure copper. Britannia silver is an alternative, hallmark-quality standard containing 95.8% silver, often used to make silver tableware and wrought plate. With the addition of germanium, the patented modified alloy Argentium Sterling silver is formed, with improved properties, including resistance to firescale.

Sterling silver jewelry is often plated with a thin coat of .999 fine silver to give the item a shiny finish. This process is called "flashing". Silver jewelry can also be plated with rhodium (for a bright, shiny look) or gold.

Silver is a constituent of almost all colored carat gold alloys and carat gold solders, giving the alloys paler color and greater hardness.[12] White 9 carat gold contains 62.5% silver and 37.5% gold, while 22 carat gold contains up to 91.7% gold and 8.4% silver or copper or a mixture of both.[12]

Historically, the training and guild organization of goldsmiths included silversmiths as well, and the two crafts remain largely overlapping. Unlike blacksmiths, silversmiths do not shape the metal while it is red-hot, but instead, work it at room temperature with gentle and carefully-placed hammer blows. The essence of silversmithing is to take a flat piece of metal and to transform it into a useful object using different hammers, stakes and other simple tools.[13]

While silversmiths specialize in, and principally work, silver, they also work with other metals, such as gold, copper, steel, and brass. They make jewelry, silverware, armor, vases, and other artistic items. Because silver is such a malleable metal, silversmiths have a large range of choices with how they prefer to work the metal. Historically, silversmiths are mostly referred to as goldsmiths, which was usually the same guild. In the western Canadian silversmith tradition, guilds do not exist; however, mentoring through colleagues becomes a method of professional learning within a community of craftspeople.[14]

Silver is much cheaper than gold, though still valuable, and so is very popular with jewelers who are just starting out and cannot afford to make pieces in gold, or as a practicing material for goldsmith apprentices. Silver has also become very fashionable, and is used frequently in more artistic jewelry pieces.

Traditionally, silversmiths mostly made "silverware" (cutlery, table flatware, bowls, candlesticks and such). Only in more recent times has silversmithing become mainly work in jewelry, as much less solid silver tableware is now handmade.

Dentistry

Silver can be alloyed with mercury, tin and other metals at room temperature to make amalgams that are widely used for dental fillings. To make dental amalgam, a mixture of powdered silver and other metals is mixed with mercury to make a stiff paste that can be adapted to the shape of a cavity. The dental amalgam achieves initial hardness within minutes but sets hard in a few hours.

Photography and electronics

Photography used 30.98% of the silver consumed in 1998 in the form of silver nitrate and silver halides. In 2001, 23.47% was used for photography, while 20.03% was used in jewelry, 38.51% for industrial uses, and only 3.5% for coins and medals. The use of silver in photography has rapidly declined, due to the lower demand for consumer color film from the advent of digital technology; since 2007, of the 907 million ounces of silver in supply, just 117.6 million ounces (13%) were consumed by the photographic sector, about 50% of the amount used in photography in 1998. By 2010 the supply had increased by about 10% to 1056.8 million ounces of which 72.7 million ounces were used in the photographic sector, a decline of 38% compared with 2007.[15]

Some electrical and electronic products use silver for its superior conductivity, even when tarnished. The primary example of this is in high quality RF connectors. The increase in conductivity is also taken advantage of in RF engineering at VHF and higher frequencies, where conductors often cannot be scaled by 6%, due to tuning requirements, e.g.cavity filters. As an additional example, printed circuits and RFID antennas can be made using silver paints,[16][17] and computer keyboards use silver electrical contacts. Silver cadmium oxide is used in high voltage contacts because it can withstand arcing.

Some manufacturers produce audio connector cables, speaker wires, and power cables using silver conductors, which have a 6% higher conductivity than ordinary copper ones of identical dimensions, but cost very much more. Though debatable, many hi-fi enthusiasts believe silver wires improve sound quality.

Small devices, such as hearing aids and watches, commonly use silver oxide batteries due to their long life and high energy to weight ratio. Another usage is high-capacity silver-zinc and silver-cadmium batteries.

Mirrors and optics

Mirrors which need superior reflectivity for visible light are made with silver as the reflecting material in a process called silvering, though common mirrors are backed with aluminium. Using a process called sputtering, silver (and sometimes gold) can be applied to glass at various thicknesses, allowing different amounts of light to penetrate. Silver is usually reserved for coatings of specialized optics, and the silvering most often seen in architectural glass and tinted windows on vehicles is produced by sputtered aluminium, which is cheaper and less susceptible to tarnishing and corrosion.[18] Silver is the reflective coating of choice for solar reflectors.[19]

Other industrial and commercial applications

Silver and silver alloys are used in the construction of high quality musical wind instruments of many types.[20] Flutes, in particular, are commonly constructed of silver alloy or silver plated, both for appearance and for the frictional surface properties of silver.[21]

Silver's catalytic properties make it ideal for use as a catalyst in oxidation reactions, for example, the production of formaldehyde from methanol and air by means of silver screens or crystallites containing a minimum 99.95 weight-percent silver. Silver (upon some suitable support) is probably the only catalyst available today to convert ethylene to ethylene oxide (later hydrolyzed to ethylene glycol, used for making polyesters)— an important industrial reaction. It is also used in the Oddy test to detect reduced sulfur compounds and carbonyl sulfides.

Silver is used to make solder and brazing alloys, and as a thin layer on bearing surfaces can provide a significant increase in galling resistance and reduce wear under heavy load, particularly against steel.

Medical

Silver ions and silver compounds show a toxic effect on some bacteria, viruses, algae and fungi, typical for heavy metals like lead or mercury, but without the high toxicity to humans normally associated with these other metals. Its germicidal effects kill many microbial organisms in vitro, but testing and standardization of silver products is difficult.[22]

Hippocrates, the "father of medicine",[23] wrote that silver had beneficial healing and antidisease properties, and the Phoenicians stored water, wine, and vinegar in silver bottles to prevent spoiling. In the early 20th century, people would put silver coins in milk bottles to prolong the milk's freshness.[24] Its germicidal effects increased its value in utensils and as jewellery. The exact process of silver's germicidal effect is still not entirely understood, although theories exist. One of these is the oligodynamic effect, which explains the effect on microorganisms, but would not explain antiviral effects.

Silver is widely used in topical gels and impregnated into bandages because of its wide-spectrum antimicrobial activity. The antimicrobial properties of silver stem from the chemical properties of its ionized form, Ag+. This ion forms strong molecular bonds with other substances used by bacteria to respire, such as molecules containing sulfur, nitrogen, and oxygen.[25] When the Ag+ ion forms a complex with these molecules, they are rendered unusable by the bacteria, depriving them of necessary compounds and eventually leading to their death.

Silver compounds were used to prevent infection in World War I before the advent of antibiotics. Silver nitrate solution use continued, then was largely replaced by silver sulfadiazine cream (SSD cream),[26] which generally became the "standard of care" for the antibacterial and antibiotic treatment of serious burns until the late 1990s.[27] Now, other options, such as silver-coated dressings (activated silver dressings), are used in addition to SSD cream. However, the evidence for the effectiveness of such silver-treated dressings is mixed, and although the evidence is promising, it is marred by the poor quality of the trials used to assess these products. Consequently, a systematic review by the Cochrane Collaboration (published in 2008) found insufficient evidence to recommend the use of silver-treated dressings to treat infected wounds.[28]

There has been renewed interest in silver as a broad-spectrum antimicrobial agent. One application has silver being used with alginate, a naturally occurring biopolymer derived from seaweed, in a range of products designed to prevent infections as part of wound management procedures, particularly applicable to burn victims.[29] In 2007, a company introduced a glass product they claimed had antibacterial properties by coating the glass with a thin layer of silver.[30] In addition, in 2007 the U.S. Food and Drug Administration (FDA) approved an endotracheal breathing tube with a fine coat of silver for use in mechanical ventilation, after studies found it reduced the risk of ventilator-associated pneumonia.[31]

Another example uses the known enhanced antibacterial action of silver by applying an electric field. In 2009, the antibacterial action of silver electrodes was found to be greatly improved if the electrodes were covered with silver nanorods.[32] The University of Missouri has found silver nanoparticles threaten benign bacteria which extract ammonia from sewage treatment systems. A serious concern is the eventual spread of the toxin into rivers, streams, lakes and ultimately the oceans.[33] A note of caution is sounded by Martin A. Philbert, professor of toxicology at the University of Michigan, Ann Arbor. "In the context of environmental health, the scientific community will have to pay close attention to those physicochemical properties of engineered nanomaterials that defeat or circumvent normal cellular processes and lend themselves to indiscriminate penetration of biological barriers, tissues, and cellular systems." [34]

Silver is commonly used in catheters. Silver alloy catheters are more effective than standard catheters for reducing bacteriuria in adults having short term catheterisation in hospitals. This meta-analysis clarifies discrepant results among trials of silver-coated urinary catheters by revealing silver alloy catheters are significantly more effective in preventing urinary tract infections than are silver oxide catheters. Though silver alloy urinary catheters cost about $6 more than standard urinary catheters, they may be worth the extra cost, since catheter-related infection is a common cause of nosocomial infection and bacteremia.[35]

Various silver compounds, devices to make homeopathic solutions and colloidal silver suspensions are sold as remedies for numerous conditions. Although most colloidal silver preparations are harmless, there are cases where excessive consumption led to argyria over a period of months or years.[36] High consumption doses of colloidal silver can result in coma, pleural edema, and hemolysis.[37]

Clothing

Silver inhibits the growth of bacteria and fungi on clothing, such as socks, so is added to reduce odors and the risk of bacterial and fungal infection. It is incorporated into clothing or shoes either by integrating silver nanoparticles into the polymer from which yarns are made or by coating yarns with silver.[38][39] The loss of silver during washing varies between textile technologies, and the resultant effect on the environment is not yet fully known.[40][41]

History

Silver has been used for thousands of years for ornaments and utensils, for trade, and as the basis for many monetary systems. Its value as a precious metal was long considered second only to gold. The word "silver" appears in Anglo-Saxon in various spellings such as seolfor and siolfor. A similar form is seen throughout the Germanic languages (compare Old High Germansilabar and silbir). The chemical symbol Ag is from the Latin for "silver", argentum (compare Greek άργυρος, árgyros), from the Indo-European root *arg- meaning "white" or "shining". Silver has been known since ancient times. Mentioned in the book of Genesis, slag heaps found in Asia Minor and on the islands of the Aegean Sea indicate silver was being separated from lead as early as the 4th millennium BC using surface mining.[16]

Recorded use of silver to prevent infection dates to ancient Greece and Rome; it was rediscovered in the Middle Ages, when it was used for several purposes, such as to disinfect water and food during storage, and also for the treatment of burns and wounds as wound dressing. In the 19th century, sailors on long ocean voyages would put silver coins in barrels of water and wine to keep the liquid potable. Pioneers in America used the same idea as they made their journey from coast to coast. Silver solutions were approved in the 1920s by the US Food and Drug Administration for use as antibacterial agents.

World War II

During World War II, the short supply of copper led to the substitution of silver in many industrial applications. The United States government loaned out silver from its massive reserve located in the West Point vaults to a wide range of industrial users. One very important use was for bus bars for new aluminum plants needed to make aircraft. During the war many electrical connectors and switches were silver plated. Another use was aircraft master rod bearings and other types of bearings. Since silver can replace tin in solder at a lower volume, a large amount of tin was freed up for other uses by substituting government silver. Silver was also used as the reflector in searchlights and other types of lights. One high-tech use of silver was for conductors at Oak Ridge National Laboratory used in calutrons to isolate uranium as part of the Manhattan project. (After the war ended the silver was returned to the vaults.)[45] Silver was used in nickels during the war to save that metal for use in steel alloys.

Occurrence and extraction

See main article: Silver mining. Silver is found in native form, as an alloy with gold (electrum), and in ores containing sulfur, arsenic, antimony or chlorine. Ores include argentite (Ag2S), chlorargyrite (AgCl) which includes horn silver, and pyrargyrite (Ag3SbS3). The principal sources of silver are the ores of copper, copper-nickel, lead, and lead-zinc obtained from Peru, Bolivia, Mexico, China, Australia, Chile, Poland and Serbia.[16] Peru, Bolivia and Mexico have been mining silver since 1546, and are still major world producers. Top silver-producing mines are Cannington (Australia), Fresnillo (Mexico), San Cristobal (Bolivia), Antamina (Peru), Rudna (Poland), and Penasquito (Mexico).[46] Top near-term mine development projects through 2015 are Pascua Lama (Chile), Navidad (Argentina), Jaunicipio (Mexico), Malku Khota (Bolivia),[47] and Hackett River (Canada).[46]

The metal is primarily produced through electrolytic copper refining, gold, nickel and zinc refining, and by application of the Parkes process on lead metal obtained from lead ores that contain small amounts of silver. Commercial-grade fine silver is at least 99.9% pure, and purities greater than 99.999% are available. In 2010, Peru was the top producer of silver (4,000 tonnes or 18% of the world's total), closely followed by Mexico (3,500 t) and China (3,000 t).[48]

Price

See main article: Silver as an investment and Silver standard. At an April 2011 price of about $49 USD per troy ounce,[49] silver is about 1/30th the price of gold. The ratio has varied from 1/15 to 1/100 in the past 100 years.

In 1980, the silver price rose to a peak for modern times of US$49.45 per troy ounce (ozt) due to market manipulation of Nelson Bunker Hunt and Herbert Hunt. Inflation adjusted to 2011 this is approximately U$D150 per troy ounce. Some time after Silver Thursday, the price was back to $10/ozt.[50] From 2001 to 2010, the price moved from $4.37 to $20.19 (Average London US$/oz).[51] According to the Silver Institute, silver's recent gains have greatly stemmed from a rise in investor interest and an increase in fabrication demand.[51] In late April 2011, silver reached an all-time high of $49.76/ozt.

In earlier times, silver has commanded much higher prices. In the early 15th century, the price of silver is estimated to have surpassed $1200 per ounce, based on 2011 dollars.[52] The discovery of massive silver deposits in the New World during the succeeding centuries has been stated as a cause for its price to have diminished greatly.

The price of silver is important in Judaic law. The lowest fiscal amount a Jewish court, or Beth Din, can convene to adjudicate a case over is a shova pruta (value of a Babylonian pruta coin). This is fixed at 1/8 of a gram of pure, unrefined silver, at market price. In a Jewish tradition, still continuing today, on the first birth-day of a first-born son, the parents pay the price of five pure-silver coins to a Kohen (priest). Today, the Israel mint fixes the coins at 117 grams of silver. The Kohen will often give those silver coins back as a gift for the child to inherit.[53]

Human exposure and consumption

Silver plays no known natural biological role in humans, and possible health effects of silver are a disputed subject. Silver itself is not toxic, but most silver salts are, and some may be carcinogenic. Silver and compounds containing it (such as colloidal silver) can be absorbed into the circulatory system and become deposited in various body tissues, leading to argyria, which results in a blue-grayish pigmentation of the skin, eyes, and mucous membranes. Although this condition does not otherwise harm a person's health, it is disfiguring and usually permanent. Argyria is rare, and mild forms are sometimes mistaken for cyanosis.[16]

Monitoring exposure

Overexposure to silver can occur in workers in the metallurgical industry, persons taking silver-containing dietary supplements, patients who have received silver sulfadiazine treatment and individuals who accidentally or intentionally ingest silver salts. Silver concentrations in whole blood, plasma, serum or urine may be measured to monitor for safety in exposed workers, to confirm the diagnosis in potential poisoning victims or to assist in the forensic investigation in a case of fatal overdosage.[54]

Use in food

Silver is used in food coloring; it has the E174 designation and is approved in the European Union. The amount of silver in the coating of dragée or as in cookie decoration is minuscule.The safety of silver for use in food is disputed. Traditional Indian dishes sometimes include the use of decorative silver foil known as vark, and in various cultures, silver dragée are used to decorate cakes, cookies, and other dessert items. The use of silver as a food additive is not approved in the United States and Australia..

Chopra, I. The increasing use of silver-based products as antimicrobial agents: a useful development or a cause for concern?. The Journal of antimicrobial chemotherapy. 59. 4. 587–90. 2007. 17307768. 10.1093/jac/dkm006.

Living Judaism: the complete guide to Jewish belief, tradition, Wayne D. Dosick - 1995 "The price was set at five shekalim (the plural of shekel, the monetary unit of the time) for each of the 273 extra firstborn (Numbers 3:47). The money was given to Aaron, the High Priest, the head of the tribe of Levi."